As integrated circuit technologies continually push toward placing more devices into smaller spaces, new photolithography methods and systems are required to resolve the increasingly smaller features. These new methods are generally called resolution enhancement techniques, and include methods such as attenuated phase shift masks and alternating phase shift masks. Attenuated phase shift masks and alternating phase shift masks were developed during the 1980's but failed to provide the manufacturable solutions to implement them successfully. The standard 6% attenuated phase shift mask technique failed to extend current lithography techniques without a reduction in wavelength, and the alternating phase shift mask technique was plagued with design, mask manufacturing, and lens aberration issues.
It was later discovered that, by increasing the background transmission of a mask, the attenuated phase shift mask technique could provide improved resolution and reduce the mask error enhancement factor, but this also introduced a new manufacturing issue. The higher transmission background required embedded chrome patches to prevent contrast from leaking into unwanted areas. The embedded patches of chrome added additional complexity and cost to implement this technique. This resolution enhancement technique is generally called embedded attenuated phase shift masks.
The chrome in an embedded attenuated phase shift mask requires additional mask and etch process steps to manufacture, and any errors in either the layout placement or the manufacture placement results in contrast leakage. As the feature pitches on the mask vary from dense to isolated feature types, an intelligent or contrast detecting algorithm is required for chrome placement. Also, the complexity of chrome repair increases for damaged chrome features that are in proximity to other chrome features. The chrome has to be placed close enough to the primary features such as contacts, vias, and trenches, to block any unwanted contrast with any high transmission scenario.
There is a need, therefore, for a mask that doesn't require chrome or another opaque material to completely block transmission, and which can still be used with high transmission backgrounds.